1. Field of the Invention
The present invention relates to element mount and barrel interfaces and more particularly to a rolling ball element mount and barrel interface for camera zoom lenses.
2. Discussion of the Related Art
Camera zoom lenses typically comprise a lens subgroup or subgroups mounted within a lens barrel for axial displacement of the subgroup or subgroups during a zooming operation. The lens subgroup can comprise, for example, several lens elements mounted on an element mount. Sliding contacts, typically comprising protrusions extending outward on an outer surface of the element mount for contacting an inner surface of the lens barrel, are used to provide for axial motion in the element mount and barrel interface. The maximum axial distance between two sliding contacts of the element mount defines its wheelbase, which must be a minimum short distance for a maximum axial displacement of the element mount. A problem with the minimum fixed wheelbase is that the short wheelbase compromises tilt control of the element mount within the barrel. Compromised tilt control leads to jamming or lock-up of the element mount within the barrel during a zooming operation.
Yet another problem with sliding contact element mount and barrel interfaces is the requirement of more than one driving force for each element mount. In other words, a sliding contact element mount and barrel interface requires at least two driving pins, mounted on each element mount and protruding through corresponding slots in the barrel, for driving each element mount. Typically, a rotatable cam ring containing cam grooves is mounted on the lens barrel for urging the driving pins, thus moving the element mounts. The requirement of more than one driving force is due to the high coefficient of friction of the sliding contacts against the lens barrel. The combination of the small wheelbase and the high coefficient of friction places design limitations upon the location and magnitude of the applied driving forces necessary for zooming.
In U.S. Pat. No. 4,709,311, a lens carrier is shown which utilizes two stationary bearing assemblies circumferentially mounted about a lens element in a lens and barrel interface. The bearing assemblies comprise rings of resilient deformable ball bearings which are positioned between the cylindrical surfaces of the lens mount and the barrel. The lens carrier of the '311 patent suffers in that with the deformable balls, the coefficient of friction is not greatly reduced, similarly as in the case of the sliding contacts, thus a substantial force for moving the lens mount axially is required. The lens carrier further suffers in that the bearing assemblies would need to be spaced closely together to achieve a maximum axial displacement of the lens within the barrel, thus compromising tilt control. Furthermore, the element mount and barrel interface of the '311 patent does not restrain the lens element from rotating relative to the barrel. Preventing the lens mount from rotating is important in a zoom lens, since, when rotated, the centerlines of each lens element of a subgroup will wobble out of a nominal position as a result of non-uniform eccentricity in each lens element. Lens elements having uniform eccentricity are available; however, their extremely high cost makes their use in a low cost zoom camera prohibitive.
An element mount supported within a barrel is essentially a three dimensional object that lies in three dimensional space and therefore has six degrees of freedom, i.e., freedom in direction of movement in lateral position (X, Y, Z) and in angular position (.THETA..sub.X, .THETA..sub.Y, and .THETA..sub.Z). Since the position of the element mount in the axial direction is generally determined via a driving means, only five of these six degrees of freedom, namely, the lateral positions (X, Y) and the angular positions (.THETA..sub.X, .THETA..sub.Y, and .THETA..sub.Z) need be considered in designing an element mount and barrel interface. An important point in properly interfacing the element mount within the barrel is to constrain its lateral (X, Y) and angular (.THETA..sub.X, .THETA..sub.Y, and .THETA..sub.Z) positions using only a minimum number of constraints, that is, to exactly constraint the element mount in the desired degrees of freedom. The element mount should be exactly constrained (i.e., not under constrained nor over constrained) in its lateral (X, Y) and angular (.THETA..sub.X, .THETA..sub.Y, and .THETA..sub.Z) positions.
A rigid body is exactly constrained in desired constrained degrees of freedom when the minimum required number of constraints against movement of the rigid body in the desired constrained degrees of freedom are used. A rigid body is under constrained in a desired constrained degree of freedom when there are insufficient constraints against motion in that desired constrained degree of freedom. Likewise, a rigid body is over constrained in a desired constrained degree of freedom when there are redundant constraints against motion in that desired constrained degree of freedom. Over constraining a desired constrained degree of freedom of a rigid body threatens the desired constraint of the rigid body in that degree of freedom since redundant constraints will compete with each other for providing the desired constraint against motion. In other words, redundant constraints make the constraint of the rigid body in that desired constrained degree of freedom unstable.
It would thus be desirable to provide an element mount and barrel interface that is simple, cost effective, and reliable. The element mount and barrel interface should allow for maximum axial displacement while minimizing the potential for jamming or tilting. Additionally, the element mount and barrel interface should exactly constrain the element mount within the barrel in all degrees of freedom except for the axial direction.